[Necessity of Measuring a Melt Level]
The Czochralski method (CZ method) is to pull a single crystal ingot from the raw material melt such as silicon in a crucible, and it is necessary to accurately measure a fluid level (hereinafter referred to as the melt level) of the raw material melt and adjust it in order to grow a crystal finely.
The accurate measurement and adjustment of the melt level by the CZ type single crystal puller are useful to control a relative position of the heat shield and the melt level or a relative position of a heater and the melt level and to promote a stable growth of a crystal.
[Adjustment of a Relative Position of the Bottom of the Heat Shield and the Melt Level]
Generally, the existing CZ type silicon single crystal puller has a heat shield (or a heat shield) which controls heat radiation from a heater and a silicon melt and also rectifies gas being introduced into the furnace and can keep constant a thermal history and an impurity concentration (such as oxygen concentration) of the pulled silicon single crystal by controlling a relative position relationship between the bottom of the heat shield and the melt surface (namely, a distance between them; hereinafter referred to as the “melt surface-heat shield gap”).
In this connection, it is essential to accurately adjust the melt surface-heat shield gap to stably produce a zero defect crystal (it is also called the “perfect crystal”) which is free from grown-in defects such as void defects, dislocation clusters or the like, and it is necessary to precisely observe the melt surface-heat shield gap.
[Melt Level Measuring Apparatus]
As an existing technology of the melt level measuring apparatus, there is an apparatus as disclosed in Japanese Patent Examined Publication No. 3-17084. This existing apparatus measures a melt level based on the principle of triangulation, and it projects a laser beam in an enlarged form and receives it in order to level the measurement variations resulting from small ripples produced on the melt surface.
But, the melt surface has a factor, which inhibits the leveling of the melt surface, independent of the above-described small ripples, and it is an obstacle to the precise measurement of the melt level. It is a meniscus which is caused in the vicinity of the crystal because of the surface tension near the growing surface of the crystal, an inclination of the melt surface in a paraboloid shape formed on the entire melt surface resulting from the rotation of the crucible and the rotation of the pulled crystal, or a phenomenon that when the heat shield is close to the melt level, the melt surface in the vicinity of the lower part of the heat shield has a concave form because of an exhaust pressure of inert gas. In the CZ type single crystal puffer having the above-described heat shield, these are especially problems to measure a melt level of the melt surface which is seen through a small gap between the heat shield and the single crystal.
In order to solve such problems, the inventors have invented and filed a patent application for a melt level measuring apparatus and a melt level measuring method which reversely use the melt surface shape which is normally formed on the melt surface to scan in the radial direction of the crucible, find a position that the irradiated laser beam is accurately guided to a photodetector, and measure a melt level based on the principle of triangulation (Japanese Patent Application No. 11-071149).
And, according to the melt level measuring apparatus and melt level measuring method of Japanese Patent Application No. 11-071149, scanning by the laser beam is performed to a range covering both the melt surface and the heat shield, and the fluid level (melt level) of the melt surface and the heat shield height (position level) can be measured respectively. And, by calculating on the basis of the measured values, a distance between the bottom of the heat shield and the melt surface (melt surface-heat shield gap) can be determined.
However, when the periphery of a single crystal and the internal circumference of the heat shield surrounding it are very close to each other and an influence of the meniscus near the crystal in the pertinent point becomes large or when the unevenness of the melt surface is lost by the application of a magnetic field to the raw material melt (silicon melt), there are problems that a position where the reflected laser beam is guided to the photodetector cannot be found even by scanning in the radial direction of the crucible, and it is difficult to measure the melt level or the melt surface-heat shield gap.